The Aletsch Glacier (German: Aletschgletscher) or Great Aletsch Glacier (German: Grosser Aletschgletscher) is the largest glacier in the Alps. It has a length of about 23 km (14 mi) and covers more than 120 square kilometres (46 sq mi) in the eastern Bernese Alps in the Swiss canton of Valais. The Aletsch Glacier is composed of three smaller glaciers converging at Concordia, where its thickness was measured by the ETH to be near 1 km (3,300 ft). It then continues towards the Rhone valley before giving birth to the Massa River.

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The Aletsch Glacier is one of the many glaciers located between the cantons of Berne and Valais on the Bernese Alps located east of the Gemmi Pass. The whole area is considered to be the largest glaciated area in western Eurasia.[1] The Fiescher and Aar glaciers lying on the east have similar extensions.

Before reaching the maximum flow, three smaller glaciers converge at Concordia (German: Konkordiaplatz):

From the western mouth flows the Great Aletschfirn, which runs along the northern foot of the Aletschhorn and Dreieckhorn. The Aletschfirn is supplied from the north by three notable firns: the Ebnefluhfirn, the Gletscherhornfirn, and the Kranzberfirn. All of these Firns have their starting points at around 3800 m. From the Ebnefluhfirns to the Konkordiaplatz, the Aletschfirn is 9 km long and is on average about 1.5 km wide. From the west, the Aletschfirn flows over the 3173 m high glacier pass, the Lötschenlücke, connecting with the Langgletscher, and then into the Lötschental valley.

From the northwestern mouth flows the Jungfraufirn. This firn in fact represents the straight continuation of the Aletsch Glacier, yet is the shortest of the three tributary glaciers. It has its origin on the southern flank of the Mönch, at the Jungfraujoch and at the eastern flank of the Jungfrau. Up to the Konkordiaplatz, the Jungfraufirn is a scarce 7 km long, and returns to flank the Kranzberg in the west and the Trugberg in the east. At its highest point, it is 2 km wide, and further down it is still a good 1 km wide.

From the northern mouth flows the Ewigschneefeld (Eternal snow field), where its starting point takes the east flank of the Mönchs. In an elbow, it flanks from Trugberg in the west and the Fiescherhorn and Grünhorn in the east, flowing on to the Konkordiaplatz. Up to here, it is about 8 km long and averages about 1.2 km wide. The mouth at the Konkordplatz it follows over a rise with a descent from 25 to 30 percent; here, the glacier is sharply split. Against the north is the Ewigschneefeld over the snow-covered pass of the Lower Mönchsjochs (3529 m high), connected with the catchment area of the Lower Grindelwald Glacier. Through the Higher Mönchsjoch (3627 m high) between the Mönch and the Trugberg stands a connection to the Jungfraufirn.

Below Concordia the glacier runs towards the valley of the Upper Rhône; on the left (east) side, near Bettmeralp, lies a glacier lake, Lake Märjelen (German: Märjelensee) (2,350 meters/7,711 feet); on the right side lie the glaciers of Mittelaletsch and Oberaletsch. At the end of the glacier, below the villages of Belalp (west) and Riederalp (east), begin the Massa river. The river flows through gorges and through the Lake Gibidum before reaching the Rhone near Brig.

The area of the Aletsch Glacier and some surrounding valleys is on the UNESCO World Heritage list, thus it is protected and the facilities are mostly restricted to the external zones. The region between Belalp, Riederalp and Bettmeralp (which is called Aletsch Region) in Valais gives access to the lower part of the glacier. The Bettmerhorn and Eggishorn are popular view points and are accessible by cable car. The Massa river can be crossed since 2008 by a suspension bridge, thus allowing hikes between the left and the right part of the glacier.

The Jungfraujoch railway station (3,450 m) gives a direct access to the upper Aletsch Glacier as well as the normal route to the Jungfrau. It can be reached only from Interlaken in the canton Berne. Glacier tours can be loved with the help of the Konkordia Hut or the Hollandia Hut, eventually reaching other glaciers in the massif.

On the Riederfurka, at 2,065 metres between Riederalp and the glacier, is located the historic Villa Cassel, former summer residence of many numerous famous and influential guests from the worlds of politics and finance. The house is now one of the centers of the environmental organization Pro Natura, which hosts a permanent exhibition about the site.[2]

Aletsch Glacier and Konkordiaplatz seen from Konkordia Hut (Sept. 2007)

Also at the mouth of the Konkordiaplatz from the east is the small but important Grüneggfirn (3 km long and averaging 600 m wide). This firn is connected in the over the glacier pass Grünhornlücke (3280 m high) to the Fiescher Glacier in the east.

From the Konkordiaplatz, the Aletsch Glacier has a width of approximately 1.5 km and moves at a rate of 180 m per year to the southeast on course with the Rhône valley, bordering the Dreieckhorn in the west and the great Wannenhorn in the east. It then takes a great right turn and bends ever closer to the southwest, running through the edge of the Eggishorn and Bettmerhorn of the Rhone valley. The lowest part of the great Aletsch Glacier is largely covered with detritus of the lateral and medial moraines. The glacier's toe currently lies about 1560 m high, far beneath the local tree line. From it springs the Massa stream, which flows though the Massa Canyon and is used to generate hydroelectric power. It continues through the upper half of the Brig, eventually entering into the Rhone.

The great Aletsch Glacier shows considerable ice cover. At the Konkordiaplatz, it has an ice cover of more than 900 m, but as it moves to the south, the greater part of the ice melts, gradually decreasing the cover to around 150 m.

The characteristically dark medial moraine, situated almost in the middle of the glacier, runs protracted in two bands from the Konkordiaplatz along the whole length to the glacier's toe-zone. This medial moraine is collected from the ice of three large ice fields, which all run together. The westernmost medial moraine has been named the Kranzbergmoräne, and the easternmost carries the name Trugbergmoräne.

The Aletsch Glacier resulted from the accumulation and compaction of snow. Glaciers generally form where snow and ice accumulation exceeds snow and ice melt. As the snow and ice thicken it reaches a point where it begins to move due to a combination of gravity and pressure of the overlying snow and ice.

During the last glacial periods, the Aletsch Glacier was much larger than now. 18,000 years ago the lower part of the ridge, between Riederalp and the glacier, was completely covered by ice. Only the summits of the Bettmerhorn, Eggishorn and the Fusshörner were above the glacier. After an important retreat, the glacier again advanced 11,000 years ago during the last glacial period. The glacier reached the Rhone valley, and its ice the Riederfurka. Remaining moraines are still visible in the Aletsch Forest.

On August 18, 2007, photographer Spencer Tunick used hundreds of naked people in a "living sculpture" on the Aletsch Glacier in a photo shoot which he claimed was intended to draw attention to global warming and the shrinking of the world's glaciers. The temperature was about 10 °C (50 °F) at the time of the photo shoot. The 600 participants on the shrinking glacier claimed that they had volunteered for Tunick (a collaboration with Greenpeace) to let the world know about the effects of global warming on the melting Swiss glaciers.[7]